Solar Thermal

Solar Hot Water

DSCN4110More than a thousand solar water heating systems have been installed in Maine over the past decade.  Owners of properly-sized systems enjoy essentially free hot water from April through October and realize significant savings on water heating during the shoulder months and on bright days in the winter.

Solar water heating systems include storage tanks and solar collectors.   In most installations, a food-grade antifreeze solution is circulated to extract solar heat trapped in solar collectors (typically located on the roof) and deliver heat to a water storage tank via an external heat exchanger or pipe coil in the tank (typically located in the basement).  The circulation pump activates any time the temperature in the collectors exceeds the temperature of water in the storage tank. While solar hot water systems can meet a household’s domestic hot water demand during summer and shoulder months, they need to be supplemented with a conventional backup system for periods of rain, snow or high usage.  Most solar thermal systems are integrated with existing domestic hot water systems or have electric backup elements that turn-on to meet hot water needs during cloudy days or in the winter months.


Courtesy the Department of Energy.

There are a number of different kinds of solar hot water systems including passive and active circulation systems.  Maine systems are typically active.  Collectors can also differ:  Maine installers install flat plate and evacuated-tube collectors.  The diagram at left includes the basic components of a solar water system.

Not all systems are appropriate for all homes and not all homes are well suited for solar. Panels are most effective when they are installed where they will receive at least eight hours of daylight without shading and positioned no more than 45 degrees from true south.  Homes with higher water use and occupancy have faster payback than homes with three or fewer occupants.   Consult a NABCEP certified solar professional listed on our vendor locator tool to find out more about what kind of system would meet your domestic hot water needs.

Taking Action

EMvendor_100x100Registered Renewable Installers – Click here to find installers in your area that sell solar systems.
EMenergyloans_100x100Get financing for your renewable project – Click here to find out about Efficiency Maine’s Home Energy Loans for renewable projects as well as energy assessments, air sealing, insulation, heat pumps and more.

Financial Example

$ 11,000 Solar Thermal Installation
$ 3,300 30% Federal tax credit
$ 6,700 net installation cost

$500/yr savings for a family of four currently using an oil boiler with a tankless coil for hot water.
13.4 year simple payback

$10,000 savings over 20-years.
Note: Actual savings may vary.

As part of a project that also addresses energy efficiency, a $10,000 solar project can be financed by Efficiency Maine for $80 per month, ($960) per year for a 15 year term at 4.99% APR fixed. Imagine locking in the price of most of your hot water use for 20 years with no upfront cost and little- to no-incremental cost over your current cost of water heating.


  1. Low-cost domestic hot water – A solar hot water system can significantly reduce domestic hot water costs since a significant portion of annual hot water demand can be produced at very low cost (simply the cost of operating circulation pumps).  Conventional boilers that provide space heat and hot water (the typical configuration in Maine homes) can be turned off during the summer when space heating isn’t needed.  Find out how much you might save on your hot water costs with Efficiency Maine’s Compare Water Heating Options tool.
  2. Predictable energy costs – Solar hot water systems can predictably produce domestic hot water with little electricity demand and can act as a hedge against future energy costs.
  3. Durability – Solar thermal systems can last 20 years or more with limited maintenance.
  4. Financing – Low-interest financing is available through Efficiency Maine for solar systems.  Homeowners may also be eligible for a 30% Federal Tax Credit.
  5. Environmental Impact – Reduce the carbon footprint of your home or business by producing domestic hot water on-site with a renewable resource.


  1.  Siting –Not all buildings or roofs are good candidates for solar hot water systems.  The most cost effective installations are on southern-facing roofs with few obstructions or shading.
  2. Back-up required — Solar water heating systems always require a backup system for cloudy days and times of increased demand.

Homeowners interested in finding out more about solar hot water systems should contact local certified installers and can click here to go to the Department of Energy’s guide to solar systems.